1. Field of the Invention
This invention relates to a multilayer piezoelectric ceramic actuator and more particularly, to an improvement of internal electrode structure thereof.
2. Description of the Prior Art
In general, the multilayer piezoelectric ceramic actuator has a laminated body that is formed by alternately laminating a plurality of piezoelectric ceramic layers and a plurality of internal electrode layers. On each of the two opposing side surfaces of the laminated body (for example, right and left side surface thereof), a glass insulator is coated onto the exposed edges of the internal electrodes on one of the opposite side surface to cover their linear edges. In this way, the glass insulators are provided on the edges of the internal electrodes arranged in the laminating direction and alternately disposed on the opposing side surfaces of the laminated body. On the opposing side surfaces of the laminated body having provided with the glass insulators, external electrode layers are respectively formed covering the glass insulators and connecting every other internal electrode on each of the opposing surfaces. The external electrodes thus formed have the lower ends respectively connected to lead wires by soldering to form a lead wire pair. The laminated body, including the external electrodes, is contained in a resin case for armoring.
With the multilayer piezoelectric ceramic actuator thus prepared, if a voltage is applied across the ends of the lead wire pair, it is applied through the external electrodes respectively to the internal electrodes. As a result, one piezoelectric ceramic layer sandwiched between internal electrodes deforms in the thickness direction thereof in accordance with the voltage applied. The deformations induced in respective ceramic layers as shown above are accumulated, and the actuator itself has a displacement induced in the laminating direction. This displacement is controlled by the voltage applied.
As explained above, the laminated type ceramic actuator has internal electrodes which are respectively sandwiched between ceramic layers. Thus, the ceramic layers are isolated with no bonding to each other. As a result, the bonding strength between the internal electrode and ceramic layer is not so large. Thus, if a tension is applied in the laminating direction, a fracture may occur at the interface between the internal electrode and the ceramic layer.
One method for solving the above problem, as disclosed, for example, in Japanese Laid-Open Patent Application No. 62-132381, is to increase the bonding strength between the internal electrode and ceramic layer by mixing a ceramic powder of the same kind as that of the ceramic layer into the internal electrode. Mixing ceramic powder into the internal electrode increases the contact area between the internal electrode and ceramic layer, utilizing the "anchor effect", thus increasing the bonding strength therebetween. Conventionally, the average particle size of the ceramic powder to be used for this purpose is considerably smaller than a thickness of the internal electrode, being not more than a quarter the thickness thereof.
According to the above-mentioned proposal, the bonding strength therebetween can be approximately doubled as compared with that obtained when not mixing the ceramic powder thereinto. In the conventional apparatus described above, a bonding strength cannot be attained that is equivalent to the strength that the ceramic layer has inherently. Thus, the problem of a fracture occurring at the interface between the internal electrode and the ceramic layer due to tension applied to the actuator in the laminating direction still remains to be solved.
Thus, an object of this invention is to provide a stack type ceramic actuator having a bonding strength between a ceramic layer and an internal electrode higher than that of being proposed as described above and a higher reliability for application of an impact force.